In Slaughterhouse-Five what detail about the hog barn shows how intense the firestorm that engulfed Dresden must have been?

There are a few different aspects of the hog barn post-firestorm that show the fire must have been extremely intense. 

"It's [the hog barn's] walls still stood, but its windows and roof were gone, and there was nothing inside but ashes and dollops of melted glass."

The fact that the windows and roof have been destroyed is one indicator of the firestorm's devastation. As well, the lack of anything substantial being left in the hog...

There are a few different aspects of the hog barn post-firestorm that show the fire must have been extremely intense. 

"It's [the hog barn's] walls still stood, but its windows and roof were gone, and there was nothing inside but ashes and dollops of melted glass."

The fact that the windows and roof have been destroyed is one indicator of the firestorm's devastation. As well, the lack of anything substantial being left in the hog barn also illustrates the fire's destructive power.

However, the most telling description present is that the windows were not just plainly destroyed, but were heated to the point of melting and becoming those "dollops" inside the hog barn. The melting point of glass is somewhere between 1400-1600°C (2552-2912°F), an obviously extraordinary temperature. The firestorm was so exceptionally intense that it emulated a glass forge--a human invention intended for the explicit purpose of heating things to unnaturally hot temperatures.